OP220_15 Datasheet, PDF (9/12 Pages) Analog Devices – Dual Micropower Operational Amplifier

OP220
In this instrumentation amplifier configuration, error due to
THREE OP AMP CONFIGURATION
CMRR effect is directly proportional to the differential CMRR A three op amp instrumentation amplifier configuration using
of the op amps. For the OP220A/E, this combined CMRR is a the OP220 and OP777 is recommended for applications requiring
minimum of 98 dB. A combined CMRR value of 100 dB and
high accuracy over a wide gain range. This circuit provides
common-mode input range of ± 2.5 V indicates a peak input-
referred error of only ± 25 mV.
Resistor matching is the other factor affecting CMRR. Defining
excellent CMR over a wide input range. As with the two op amp
instrumentation amplifier circuits, tight matching of the two op
amps provides a real boost in performance.
Ad as the differential gain of the instrumentation amplifier and
assuming that R1, R2, R3 and R4 are approximately equal (R_N
will be the nominal value), then CMRR will be approximately
R1
V_O = V_D
1 + ^2R1
R0
A_D divided by 4DR/R_N. CMRR at differential gain of 100 would
be 88 dB with resistor matching of 0.1%. Trimming R1 to make
the ratio R3/R4 equal to R2/R1 will directly raise the CMRR
V_CM – 1/2 V_{D –}
R2
A1
V1
1/2
R2
V+
OP777
until it is limited by linearity and resistor stability considerations.
The high open-loop gain of the OP220 is very important in
achieving high accuracy in the two-op-amp instrumentation
amplifier configuration. Gain error can be approximated by:
E _GainError
=
1
1+
A_D
A₀₂
, ^AD
2A₀₁A₀₂
<1
T _{where AD is the instrumentation amplifier differential gain and}
A₀₂ is the open-loop gain of op amp A2. This analysis assumes
equal values of R1, R2, R3, and R4. For example, consider an
E ^{OP220 with A}02 ^{of 700 V/mV. If the differential gain A}D ^were
set to 700, the gain error would be 1/1.001 which is approxi-
mately 0.1%.
Another effect of finite op amp gain is undesired feedthrough of
L _{common-mode input. Defining A01 as the open-loop gain of op}
amp A1, then the common-mode error (CME) at the output
due to this effect will be approximately:
O _CME
=
2A_D
1+
A_D
A₀₁
1
A₀₁ ^VCM
S _{For AD/A01, < 1, this simplifies to (2 AD/A01) Ï« VCM. If the op}
amp gain is 700 V/mV, V_CM is 2.5 V, and A_D is set to 700, then
the error at the output due to this effect will be approximately 5 mV.
The OP220 offers a unique combination of excellent dc perfor-
B _{mance, wide input range, and low supply current drain that is}
O ^{particularly attractive for instrumentation amplifier design.}
OP220
R0
A3
V_O
V_D
R1
V+
V_CM + 1/2 V_D ⁺
R2
A2
V–
V2
1/2
R2
OP220 ^V–
Figure 5. Three Op Amp Instrumentation Amplifier Using
OP220 and OP777
A simplified schematic is shown in Figure 2. The input stage
(A1 and A2) serves to amplify the differential input V_D without
amplifying the common-mode voltage V_CM. The output stage
then rejects the common-mode input. With ideal op amps and
no resistor matching errors, the outputs of each amplifier will be:
V1 =
-ÊËÁ1 +
2R1ˆ
R_O ¯˜
V_D
2
+ V_CM
V
2
=
Ê
ËÁ
1
+
2R1ˆ
R_O ¯˜
V_D
2
+ V_CM
V_O
=V 2 -V1 =
Ê
ËÁ
1
+
2R1ˆ
R_O ¯˜
V_D
V_O = A_DV_D
The differential gain A_D is 1 + 2R1/RO and the common-mode
input V_CM is rejected.
This three op amp instrumentation amplifier configuration using an
OP220 at the input and an OP777 at the output provides excellent
performance over a wide gain range with very low power consump-
tion. A gain range of 1 to 2,000 is practical and CMR of over
120 dB is readily achievable.
REV. A
–9–